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Questions about transistors in upcoming amplifier

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kraftwerk1976
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Questions about transistors in upcoming amplifier

Post by kraftwerk1976 » Thursday, 18 August 2016, 9:38 AM

--Hi, guys. I am saving up money to have a builder make me an amplifier. What I had planned for him to build me, was a 16 transistor mobile (but will be used in the house, and will rely on an outboard power supply) with MaCom MRF421 transistors in it. As you know, the MRF421 is a 12V, 100 watt transistor. However, I got to thinking about having the builder use MaCOM MRF422 transistors, which are 28V, 150 watt rated transistors. Now, I know that in order to use this amplifier at its rating of 28V, I would obviously need to get a 28V power supply. That would not be a problem. If I were to have this amplifier built, with 16 MRF422 transistors, and I decided to use this amplifier at 14V, though, what kind of output would I see from it? Doing the math, MRF422 are rated at 150 watts on MaCom's site. Multiply that times 16, you get 2400 watts...and yes, that would be if the amplifier would be used at 28V. So, if I used my 14V power supply on the same amplifier, what kind of watts would I see? Exactly half (1200 watts)? Not sure what would happen exactly if you cut the voltage in half.

--Now, If I were to have him make make my original idea of using 16, MRF421 transistors, at least according to the manufacturer's ratings @100 watts each, that would be roughly 1600 watts, which is obviously more than the 1200 watts a 16 transistor MRF422 would put out at 14V (assuming the wattage drops exactly in half when dropping exactly half in your voltage from the power supply.

--So, can anyone tell me if indeed transistors drop to approximately half power when the voltage is halved (in this case from 28V to 14V)? Just looking at RF Parts, the MRF421, rounded up, is $76/transistor. The price of the MRF422 is $76/transistor as well. So no extra costs involved/no savings involved for this project, at least when it comes to the transistors. However, assuming that when voltage halves, and the transistors put out half their power as well, the MRF422 should be close to "loafing". Plus, I will always have the option to run the amplifier with MRF422s in it to its rated 28V specs., and have even more output power compared to the MRF421s.

--Sorry for the long question(s), and while I wish I knew a LOT more about RF power than what I do, I'm not ashamed to say I am ignorant in electronics, lol...but I guess being "ignorant" in something isn't something to necessarily be proud of, lol. But, anyway, I truly appreciate you guys' help. You seem to have the most knowledge here. I know the builder will have no problem building a 16 transistor amplifier with MRF421s in it, but not sure if he will go for building one with 16 MRF422s in it (because I just don't know how much more adjusting/engineering would be involved). Thanks.


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Re: Questions about transistors in upcoming amplifier

Post by Crusher » Thursday, 18 August 2016, 18:13 PM

The problem with running a 28v device at 14V, unless builder tunes it up for 14v. The box will heat up a lot quicker and not put out the power you hoped for. Changing the voltage , changes the tune. This is because the voltage is different and will affect how the box will perform. Meaning not just input and output tune but possibly the capacitors across the transformers as well. Expect it to be a power hog



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Re: Questions about transistors in upcoming amplifier

Post by kraftwerk1976 » Thursday, 18 August 2016, 20:35 PM

Thank you very much for the answer. So, basically have him build one or the other, meaning, build the amp. with 16 MRF421s (or whatever amount of transistors I finally decide on) and expect it to run at roughly 14V. OR, have him build me an amplifier with 16 MRF422s, BUT only expect to run the amplifier, while still having a good tune on it, at about 28V on it. Pretty sure that's what you meant, anyhow. I was SO hoping that I could have an MRF422 amplifier with all this range of output all by only turning down the power supply from 28V, down to 20, down to 16, and finally to 13.8V. But, since I don't want to mess with tubes in my life right now, and nobody could point me in the direction of someone making a good LDMOS amplifier, I decided to go with MaCom because the are for sure good quality.

--One last thing about using a transistor to work over a range of 14V-28V. I was watching a video from He-Who-Must-Not-Be-Named a bit ago. He was using two 2SC2510 transistors, and it seemed to work just fine. I couldn't ask HIM any questions because he disabled comments on the You Tube page I was watching it from. It's just all so confusing...but I figure it will be just fine if I go like I first wanted and just have the other fellow build me that amplifier with 16 MRF421s. Thanks again for taking the time to answer my ignorant questions.


For mobile: Antenna: President Maryland USA Radio: President Johnson II USA

For base: Antenna: MaCo V58. Radio: President P400, RCI-2950DX, Cobra29
Black Chrome LTD Classic with a Loudmouth audio module inside the radio. Works well.

For amplifier in the house: Palomar Elite 450HD run conservatively with an added fan.

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Re: Questions about transistors in upcoming amplifier

Post by Crusher » Friday, 19 August 2016, 2:11 AM

Either way it will work. But if you use 28v pills on 14 volts. It may need slight retuning to keep up efficiency. By the way, the 2sc2510A is a better device.

-- August 19 2016 --

The **Censored** I think you hinted about. He should be able to explain why the devices act different with different voltages. Think of those input and output transformers as tank circuits in a tube amplifier. What happens when you load if differently with say different voltage or drive. The circuit has to be returned to get the most efficiency out of amp



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Re: Questions about transistors in upcoming amplifier

Post by MDYoungblood » Saturday, 20 August 2016, 11:42 AM

Crusher has a bunch of good answers and advice, he is one to listen too.
My question is where will it be used the most, the 28v transistors would be a better build for a base unit and would have the builder make a power supply that will handle the amp, (if he can build an amp then he should be able to build the PS), then just get a smaller 14v amp (a 4 pill) for the vehicle, have less problems for you and the vehicle, (RF is a terrible thing to waste). Just remember clean in equals clean out.

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Re: Questions about transistors in upcoming amplifier

Post by -=PEAKABOO=- » Thursday, 10 November 2016, 16:44 PM

The higher voltage device ran at 28 volts will be better for base use.


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Re: Questions about transistors in upcoming amplifier

Post by Crusher » Thursday, 10 November 2016, 18:13 PM

Glad to see Peekaboo still around




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